# Hardhat Network

Hardhat comes built-in with Hardhat Network, a local Ethereum network designed for development. It allows you to deploy your contracts, run your tests and debug your code.

# How does it work?

It mines a block with each transaction that it receives, in order and with no delay.
It's backed by the ethereumjs-vm EVM implementation, the same one used by ganache, Remix and Ethereum Studio.
It supports the following hardforks:
- byzantium
- constantinople
- petersburg
- istanbul
- muirGlacier

# How can I use it?

Hardhat will always spin up an instance on startup when defaultNetwork is empty or set to hardhat. It's the default behavior.
It can be used to run tests, in the console, scripts, and tasks.
Plugins (ethers.js, web3.js, Waffle, Truffle, etc) connect directly to the provider.
There's no need to make any changes to your tests or scripts.
It's simply another network and it can be used with --network.

# Connecting to Hardhat Network from wallets and other software

Hardhat Network can run in a standalone fashion so that external clients can connect to it. This could be MetaMask, your Dapp front-end, or a script. To run Hardhat Network in this way, run:

npx hardhat node

It will start Hardhat Network, and expose it as a JSON-RPC and WebSocket server.

Then, just connect your wallet or application to http://localhost:8545.

If you want to connect Hardhat to this node, you only need to run it using --network localhost.

# Solidity stack traces

Hardhat Network has first-class Solidity support. It always knows which smart contracts are being run, what they do exactly and why they fail.

If a transaction or call fails, Hardhat Network will throw an exception. This exception will have a combined JavaScript and Solidity stack trace: stack traces that start in JavaScript/TypeScript up to your call to the contract, and continue with the full Solidity call stack.

This is an example of a Hardhat Network exception using TruffleContract:

Error: Transaction reverted: function selector was not recognized and there's no fallback function
  at ERC721Mock.<unrecognized-selector> (contracts/mocks/ERC721Mock.sol:9)
  at ERC721Mock._checkOnERC721Received (contracts/token/ERC721/ERC721.sol:334)
  at ERC721Mock._safeTransferFrom (contracts/token/ERC721/ERC721.sol:196)
  at ERC721Mock.safeTransferFrom (contracts/token/ERC721/ERC721.sol:179)
  at ERC721Mock.safeTransferFrom (contracts/token/ERC721/ERC721.sol:162)
  at TruffleContract.safeTransferFrom (node_modules/@nomiclabs/truffle-contract/lib/execute.js:157:24)
  at Context.<anonymous> (test/token/ERC721/ERC721.behavior.js:321:26)

The last two lines correspond to the JavaScript test code that executed a failing transaction. The rest is the Solidity stack trace. This way you know exactly why your tests aren't passing.

# Automatic error messages

Hardhat Network always knows why your transaction or call failed, and uses this information to make debugging your contracts easier.

When a transaction fails without a reason, Hardhat Network will create a clear error message in the following cases:

Calling a non-payable function with ETH
Sending ETH to a contract without a payable fallback or receive function
Calling a non-existent function when there's no fallback function
Calling a function with incorrect parameters
Calling an external function that doesn't return the right amount of data
Calling an external function on a non-contract account
Failing to execute an external call because of its parameters (e.g. trying to send too much ETH)
Calling a library without DELEGATECALL
Incorrectly calling a precompiled contract
Trying to deploy a contract that exceeds the bytecode size limit imposed by EIP-170

# `console.log`

Hardhat Network allows you to print logging messages and contract variables calling console.log() from your Solidity code. You can see an example in the Sample Project. Follow the steps in Quick Start to try it out.

You can use it in calls and transactions. It works with view functions, but not in pure ones.
It always works, regardless of the call or transaction failing or being successful.
To use it you need to import hardhat/console.sol.
You can call console.log with up to 4 parameters in any order of following types:
- uint
- string
- bool
- address
There's also the single parameter API for the types above, and additionally bytes, bytes1.. up to bytes32:
- console.logInt(int i)
- console.logUint(uint i)
- console.logString(string memory s)
- console.logBool(bool b)
- console.logAddress(address a)
- console.logBytes(bytes memory b)
- console.logByte(byte b)
- console.logBytes1(bytes1 b)
- console.logBytes2(bytes2 b)
- ...
- console.logBytes32(bytes32 b)
console.log implements the same formatting options that can be found in Node.js' console.log, which in turn uses util.format.
- Example: console.log("Changing owner from %s to %s", currentOwner, newOwner)
It works with any library: ethers.js, web3.js, waffle, truffle-contract, etc.
console.log is implemented in standard Solidity and then detected in Hardhat Network. This makes its compilation work with any other tools (like Remix, Waffle or Truffle).
console.log calls can run in other networks, like mainnet, kovan, ropsten, etc. They do nothing in those networks, but spend a minimal amount of gas.

# Mainnet forking

The Hardhat Network is empty by default, except for some accounts with an initial balance. But sometimes it's more useful to have a local network that simulates the state of the mainnet. This is what forking is for.

To fork from the mainnet you need the URL of a node to connect to. For example, using Infura, you can start a local node that forks the mainnet with this command:

npx hardhat node --fork https://eth-mainnet.alchemyapi.io/v2/<key>

where you have to replace <projectId> with your Infura project id.

After doing this, you can do anything in your node that you can do with a non-forked Hardhat Network: see console logs, get stack traces or use the default accounts to deploy new contracts.

If you want this to be the default behavior, you can do it in your Hardhat config:

networks: {
  hardhat: {
    forking: {
      url: "https://mainnet.infura.io/v3/<projectId>"
    }
  }
}

This means that if you execute a task that uses the Hardhat Network, that task will start a forked node and run on it.

There are other things you can do with a forked Hardhat Network, check our guide to learn more.

# Logging

Hardhat Network uses its tracing infrastructure to offer rich logging that will help you develop and debug smart contracts.

For example, a successful transaction and a failed call would look like this:

eth_sendTransaction
  Contract deployment: Greeter
  Contract address: 0x8858eeb3dfffa017d4bce9801d340d36cf895ccf
  Transaction: 0x7ea2754e53f09508d42bd3074046f90595bedd61fcdf75a4764453454733add0
  From: 0xc783df8a850f42e7f7e57013759c285caa701eb6
  Value: 0 ETH
  Gas used: 568851 of 2844255
  Block: #2 - Hash: 0x4847b316b12170c576999183da927c2f2056aa7d8f49f6e87430e6654a56dab0

  console.log:
    Deploying a Greeter with greeting: Hello, world!

eth_call
  Contract call: Greeter#greet
  From: 0xc783df8a850f42e7f7e57013759c285caa701eb6

  Error: VM Exception while processing transaction: revert Not feeling like it
      at Greeter.greet (contracts/Greeter.sol:14)
      at process._tickCallback (internal/process/next_tick.js:68:7)

This logging is enabled by default when using Hardhat Network's node (i.e. npx hardhat node), but disabled when using the in-process Hardhat Network provider. See Hardhat Network's config to learn more about how to control its logging.

# Hardhat Network initial state

Hardhat Network is initialized by default in this state:

A brand new blockchain, just with the genesis block.
20 accounts with 10000 ETH each, generated with the mnemonic "test test test test test test test test test test test junk". Their addresses are:
- 0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266
- 0x70997970C51812dc3A010C7d01b50e0d17dc79C8
- 0x3C44CdDdB6a900fa2b585dd299e03d12FA4293BC
- 0x90F79bf6EB2c4f870365E785982E1f101E93b906
- 0x15d34AAf54267DB7D7c367839AAf71A00a2C6A65
- 0x9965507D1a55bcC2695C58ba16FB37d819B0A4dc
- 0x976EA74026E726554dB657fA54763abd0C3a0aa9
- 0x14dC79964da2C08b23698B3D3cc7Ca32193d9955
- 0x23618e81E3f5cdF7f54C3d65f7FBc0aBf5B21E8f
- 0xa0Ee7A142d267C1f36714E4a8F75612F20a79720
- 0xBcd4042DE499D14e55001CcbB24a551F3b954096
- 0x71bE63f3384f5fb98995898A86B02Fb2426c5788
- 0xFABB0ac9d68B0B445fB7357272Ff202C5651694a
- 0x1CBd3b2770909D4e10f157cABC84C7264073C9Ec
- 0xdF3e18d64BC6A983f673Ab319CCaE4f1a57C7097
- 0xcd3B766CCDd6AE721141F452C550Ca635964ce71
- 0x2546BcD3c84621e976D8185a91A922aE77ECEc30
- 0xbDA5747bFD65F08deb54cb465eB87D40e51B197E
- 0xdD2FD4581271e230360230F9337D5c0430Bf44C0
- 0x8626f6940E2eb28930eFb4CeF49B2d1F2C9C1199

To customise it, take a look at the configuration section.

# JSON-RPC methods support

# Supported methods

eth_accounts
eth_blockNumber
eth_call
eth_chainId
eth_coinbase
eth_estimateGas
eth_gasPrice
eth_getBalance
eth_getBlockByHash
eth_getBlockByNumber
eth_getBlockTransactionCountByHash
eth_getBlockTransactionCountByNumber
eth_getCode
eth_getFilterChanges
eth_getFilterLogs
eth_getLogs
eth_getStorageAt
eth_getTransactionByBlockHashAndIndex
eth_getTransactionByBlockNumberAndIndex
eth_getTransactionByHash
eth_getTransactionCount
eth_getTransactionReceipt
eth_mining
eth_newBlockFilter
eth_newFilter
eth_newPendingTransactionFilter
eth_pendingTransactions
eth_sendRawTransaction
eth_sendTransaction
eth_signTypedData
eth_sign
eth_subscribe
eth_syncing
eth_uninstallFilter
eth_unsubscribe
net_listening
net_peerCount
net_version
web3_clientVersion
web3_sha3

# Hardhat network methods

hardhat_addCompilationResult – Add information about compiled contracts
hardhat_impersonateAccount – see the Mainnet Forking guide
hardhat_stopImpersonatingAccount – see the Mainnet Forking guide
hardhat_reset – see the Mainnet Forking guide
hardhat_setLoggingEnabled – Enable or disable logging in Hardhat Network

# Special testing/debugging methods

evm_increaseTime – same as Ganache.
evm_mine – same as Ganache
evm_revert – same as Ganache.
evm_snapshot – same as Ganache.
evm_setNextBlockTimestamp - this method works like evm_increaseTime, but takes the exact timestamp that you want in the next block, and increases the time accordingly.

# Unsupported methods

eth_compileLLL
eth_compileSerpent
eth_compileSolidity
eth_getCompilers
eth_getProof
eth_getUncleByBlockHashAndIndex
eth_getUncleByBlockNumberAndIndex
eth_getUncleCountByBlockHash
eth_getUncleCountByBlockNumber
eth_getWork
eth_hashrate
eth_protocolVersion
eth_signTransaction
eth_submitHashrate
eth_submitWork

# Limitations

# Supported Solidity versions

Hardhat Network can run any smart contract, but it only understands Solidity 0.5.1 and newer.

If you are compiling with an older version of Solidity, or using another language, you can use Hardhat Network, but Solidity stack traces won't be generated.

# Solidity optimizer support

Hardhat Network can work with smart contracts compiled with optimizations, but this may lead to your stack traces' line numbers being a little off.

We recommend compiling without optimizations when testing and debugging your contracts.

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Last Updated: 10/22/2020, 7:44:27 PM

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